log4j-core/src/main/java/org/apache/logging/log4j/core/util/CoarseCachedClock.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache license, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the license for the specific language governing permissions and
 * limitations under the license.
 */
package org.apache.logging.log4j.core.util;

import java.util.concurrent.locks.LockSupport;

/**
 * This Clock implementation is similar to CachedClock. It is slightly faster at
 * the cost of some accuracy.
 */
public final class CoarseCachedClock implements Clock {
    private static volatile CoarseCachedClock instance;
    private static final Object INSTANCE_LOCK = new Object();
    // ignore IDE complaints; volatile long is fine
    private volatile long millis = System.currentTimeMillis();

    private final Thread updater = new Log4jThread("CoarseCachedClock Updater Thread") {
        @Override
        public void run() {
            while (true) {
                millis = System.currentTimeMillis();

                // avoid explicit dependency on sun.misc.Util
                LockSupport.parkNanos(1000 * 1000);
            }
        }
    };

    private CoarseCachedClock() {
        updater.setDaemon(true);
        updater.start();
    }

    /**
     * Returns the singleton instance.
     *
     * @return the singleton instance
     */
    public static CoarseCachedClock instance() {
        // LOG4J2-819: use lazy initialization of threads
        CoarseCachedClock result = instance;
        if (result == null) {
            synchronized (INSTANCE_LOCK) {
                result = instance;
                if (result == null) {
                    instance = result = new CoarseCachedClock();
                }
            }
        }
        return result;
    }

    /**
     * Returns the value of a private long field that is updated by a background
     * thread once every millisecond. Because timers on most platforms do not
     * have millisecond granularity, the returned value may "jump" every 10 or
     * 16 milliseconds.
     * @return the cached time
     */
    @Override
    public long currentTimeMillis() {
        return millis;
    }
}